The present invention relates to a radiation-polymerizable composition. More particularly, this invention relates to a radiation-polymerizable composition which may be employed to produce an improved photographic element such as a lithographic printing plate, or color proofing film or photoresist, especially dry film photoresists.
The use of radiation-polymerizable coatings in the graphic arts is well known. Typically, the composition of such coatings can include a polymeric constituent, which may itself be radiation-polymerizable, and a photosensitizer composition. Upon selected exposure of this coating to imaging energies, the radiation-polymerizable species within the composition would either itself undergo a reaction or promote a reaction or degradation of one or more of the other components of the composition. It is, of course, readily appreciated that such reaction is only of value in such a system where it is essentially confined, or limited, to those areas of the composition impinged upon by such imaging energies. A negative working image thus created within the coating can then be "developed" by selective removal of the non-exposed components of the film or layer which have not been subjected to imaging energies. The ability to create such selective changes in coatings prepared from a radiation-polymerizable composition has been appreciated for some time. The basic difference in the various approaches in the formulation of lithographic printing plates from radiation-polymerizable compositions has been in the search for a system which is prepared from relatively inexpensive ingredients, does not require prolonged imaging energies (has a high quantum efficiency), is able to undergo an increased number of press runs and results in the creation of high resolution images within the composition which can be manifested without prolonged and elaborate development and, significantly, may be developed with a composition which does not necessarily contain organic solvents and which need contain only a minor amount of salts and surfactants.
Most such lithographic printing plates comprise a metal substrate which is coated with a light sensitive diazonium compound in admixture with suitable binding resins, photoinitiators, photopolymerizable compositions, colorants, stabilizers, exposure indicators, surfactants and the like. On the other hand, color proofing films are frequently comprised of the foregoing compositions disposed on a transparent substrate such as polyethylene terephthalate.
Photographic processes are known which use photopolymers. There are various types of photographic color proofing methods; for example, the surprint type and the overlay type. In the overlay type of color proofing method, an independent transparent plastic support is used for producing a print of each color separation film by applying a photosensitive solution of the corresponding color, and a plurality of such supports carrying prints of corresponding colors are then superposed upon each other on a white sheet to produce a color proofing sheet. The overlay type of color proofing method has the disadvantage that the superposed plastic supports tend to darken the color proofing sheet and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press or a proof press. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or three colors in register.
In the surprint type of color proofing method, a color proofing sheet is prepared by successively producing prints of different colors from different color separation films, respectively, by utilizing a single opaque support by applying toners, or by applying photosensitive solutions or coatings of photopolymers of corresponding colors on the opaque support in succession. Examples of some of these approaches are: U.S. Pat. No. 3,671,236 and U.S. Pat. No. 3,136,637. An advantage of the surprint type of color proof is that the color saturation is not influenced by the plastic support. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system. Various processes for producing copies of an image embodying a photopolymerization and thermal transfer techniques are known. See U.S. Pat. Nos. 3,060,023, 3,060,024, 3,060,025, 3,481,736 and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is exposed, imagewise, to a process transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the said elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter adheres, preferentially, to the clear unpolymerized material. The lamination, exposure and development steps are carried out for the respective colors, in sequence, thus making these processes time consuming.
Photopolymerizable imaging systems typically comprise an unsaturated monomer, a free-radical initiator which is activated by active radiation, and binder resins, colorants and the other optional additives as hereinbefore mentioned.
In the production of free-radical initiated photopolymerizable coatings, it is widely recognized that such reactions are subject to a quenching phenomenum by triplet oxygen of the free-radicals formed by irradiating the initiator. The premature return of the free-radicals to the energy ground state precludes the required energy transfer necessary to effect the polymerization reaction.
An art recognized method useful in preventing triplet oxygen quenching of radiation formed free-radicals is overcoating the base coating with a watersoluble polymeric resin. Such resins must be transparent, film-forming polymers which substantially act as oxygen barrier layers and which are inert and incompatible with all ingredients comprising the base coating.
The present invention improves upon the foregoing class of compositions by providing a photopolymerizable composition which does not require such an oxygen barrier. The composition employs a polymerizable monomer which is poly butane diol diacrylate in admixture with a polymerization initiator.